(a) Field of the Invention
The present invention relates to a method for preparing syndiotactic polypropylene and propylene copolymers. More specifically, it relates to a method for preparing syndiotactic polypropylene and propylene copolymers having a desirable molecular weight and a high tacticity.
(b) Description of the Prior Art
Syndiotactic polypropylene has been known for a long period of time. However, such polypropylene prepared by polymerization at a low temperature in the presence of a conventional catalyst comprising a vanadium compound, an ether and an organic aluminum compound was poor in syndiotacticity. Therefore, it was scarcely considered to have characteristic properties as syndiotactic polypropylene.
A polypropylene having a good syndiotacticity, i.e., a syndiotactic pentad fraction (rrrr) of more than 0.7 in accordance with .sup.13 C-NMR has been discovered for the first time by J. A. Ewen et al. which can be obtained by polymerizing propylene by the use of a polymerization catalyst comprising a transition metal (Hf or Zr) compound having asymmetric ligands and methylaluminoxane (J. Amer. Chem. Soc., Vol. 110, p. 6255-6256, 1988).
According to the above-mentioned J. A. Ewen et al. method, the above-mentioned catalyst has good activity per transition metal, and syndiotactic polypropylene having a good syndiotacticity can be obtained. In addition, when propylene is copolymerized with another olefin in accordance with the above-mentioned method, a propylene-olefin copolymer having a substantially syndiotactic structure can be obtained.
In the aforesaid polymerization or copolymerization, the molecular weight of the obtained polymer or copolymer depends upon a polymerization temperature. However, there is the problem that the control of the molecular weight cannot be achieved sufficiently by hydrogen which is used for molecular weight control in manufacturing the polypropylene or propylene copolymer by the use of a conventional Ziegler catalyst.